As the Internet continues to grow exponentially, the bandwidth requirements of the associated networks comprising the Internet similarly continue to grow. The transmission of large quantities of multimedia data, for example, including both audio and video data, has contributed to what has become a seemingly universal demand for increased network bandwidth. In addition to requiring large amounts of bandwidth, real-time multimedia data also tends to be isochronous, in that it is to be received at a destination device at the same rate with which it was transmitted from the sending device. If such real-time multimedia data is delayed during transmission between the source device and the destination device, the resulting image and/or sound quality of the multimedia “stream” may be adversely affected.
When a network segment located between a source device and a destination device becomes unavailable, data en route from the source device to the destination device has to be rerouted so as to bypass the unavailable network segment. Such rerouting of data due to unexpected network outages may add significant delay to the total data transmission time. One cause for unexpected network outages and associated data transmission delays may be attributed to excavation accidents where one or more network segments are severed. Conventional copper networks, for example, tend to not be very resilient and are quite susceptible to such accidental severing or “backhoe fade.”
New technologies are continually being introduced to address the various demands for increased network bandwidth and improved resiliency. Optical networking is one such technology. In general, optical networks utilize optical glass wires or “fibers” to transmit data in the form of light pulses along the fiber. Among other features, optical fiber is capable of carrying much more information than conventional copper wire and is generally not subject to electromagnetic interference as is copper wiring.
Synchronous Optical Network (hereinafter “SONET”) is a technology that combines the high-bandwidth network capacity of optical fiber with the resiliency of automatic protection switching. The SONET optical interface is specified in the American National Standard for Telecommunications—Synchronous optical network (SONET)—Basic description including multiplex structures, rates, and formats, ANSI T1.105-1995; and SONET Automatic Protection Switching (hereinafter “APS”) is specified in the American National Standard for Telecommunications—Synchronous optical network (SONET)—Automatic Protection Switching, ANSI T1.105.01-1998. Synchronous Digital Hierarchy (hereinafter “SDH”) is the international equivalent of SONET and is standardized by the International Telecommunications Union (ITU). Although SDH may not be specifically referred to herein, the concepts described with respect to SONET nonetheless apply equally to SDH.
The APS protocol describes an architecture in which data signals travelling across one signal path may be automatically switched to another signal path due to a variety of circumstances such as signal degradation or line failure. For example, assume a router is connected to a signal multiplexor by both a working line (i.e. primary circuit) and a protection line (i.e. backup circuit). If the working line were to fail, or signal quality on the working line were to degrade, APS would perform an automatic switchover so that signals would travel to and from the router via the protection line rather than the working line, thereby maintaining communication between the multiplexor and the router.
Several different automatic protection switching schemes are addressed by the APS specification. One such switching scheme is protection switching for linear 1+1 topologies. Linear APS is used to protect tributary SONET lines which connect routers to Add-Drop multiplexors (hereinafter “ADM”), whereas ring APS architectures protect the lines between equipment comprising the SONET ring itself. FIG. 1 illustrates an exemplary linear APS 1+1 architecture according to the prior art. As shown in FIG. 1, each router is coupled to the ADM and SONET ring by both a working line and a protection line. Accordingly, if either working line were to fail, for example, the corresponding protection line could be used in order to maintain communication with the SONET ring.
Each working line and protection line pair may be coupled to an associated router through either a single SONET interface module or a pair of SONET interface modules. In FIG. 1, for example, each working/protection line pair is coupled to an associated router through a pair of interface modules, however, a single module implementation may also be utilized. While the single module implementation protects against line failures, the two module implementation protects against both line and interface module failures. Neither implementation, however, protects against interruptions in network communication due to an entire router failure.